Thread wound golf ball

ABSTRACT

The present invention provides a solid center thread wound golf ball having long flight distance, while maintaining the characteristics peculiar to the conventional thread wound golf ball, i.e. good shot feel and good controllability. The present invention related to a solid center type thread wound golf ball comprising  
     (a) a solid center composed of an inner center and a center outer layer formed on the inner center,  
     (b) a thread rubber layer formed on the solid center, and  
     (c) a cover covering the thread rubber layer, wherein the inner center has a diameter of 20 to 35 mm, the solid center has a diameter of 30 to 38 mm, and a specific gravity of the center outer layer is larger than that of the inner center by 0.1 to 0.4.

FIELD OF THE INVENTION

[0001] The present invention relates to a thread wound golf ball comprising a solid center (a solid center type thread wound golf). More particularly, it relates to a solid center type thread wound golf ball having long flight distance, while maintaining the characteristics peculiar to the conventional thread wound golf ball, i.e. good shot feel and good controllability.

BACKGROUND OF THE INVENTION

[0002] Many golf balls are commercially selling, but they are typically classified into solid golf balls such as two-piece golf ball and three-piece golf ball, and thread wound golf balls. The thread wound golf ball consists of a thread wound core comprising a thread wound layer and a cover covering on the thread wound core. The thread wound golf ball, when compared with the solid golf ball, has better shot feel at the time of hitting and better spin performance. The thread wound golf ball is generally approved of or employed by high level golfers, especially professional golfers, who regard the characteristics as most important. On the other hand, the thread wound golf ball is inferior in flight distance to the solid golf ball.

[0003] In the thread wound golf balls, there are two types depending on the type of center, on which thread rubber is wound to form thread wound core, such as a solid center type and liquid center type. The liquid center is composed of a hollow rubber sphere and liquid encapsulated in the sphere. The solid center is formed from unvulcanized rubber material. In the solid center type thread wound golf ball, the rebound characteristics of the solid center have effect on those of the golf ball. Therefore the solid center type thread wound golf ball, when compared with the liquid center type thread wound golf ball, is superior in flight distance. However, since the thread wound golf ball is generally inferior in flight distance as described above, it is required to increase the fight distance of the thread wound golf ball, while maintaining the characteristics of having good shot feel and good spin performance.

[0004] It has been proposed to improve the flight distance of the solid center type thread wound golf ball by various means. For example, in Japanese Patent Kokai Publication No. 253236/1997, a thread wound golf ball comprising a solid center of two-layer structure composed of an inner center and an outer center, of which the inner center has a Shore D hardness of 10 to 35 and a diameter of 18 to 28 mm, the outer center has a Shore D hardness of 36 to 63, the hardness of the outer center is larger than that of the inner center by not less than 5, and the solid center has a diameter of 29 to 39.5 mm, is described. The thread wound golf ball has longer flight distance than the conventional thread wound golf ball, but the technical effect of improving the flight distance is not sufficiently obtained. In addition, since the outer portion of the golf ball having high hardness has large thickness, the shot feel is not sufficient. A solid center type thread wound golf ball having long flight distance, while maintaining the good shot feel and good controllability of the conventional thread wound golf ball, has been required.

OBJECTS OF THE INVENTION

[0005] A main object of the present invention is to provide a solid center type thread wound golf ball having long flight distance, while maintaining the characteristics peculiar to the conventional thread wound golf ball, i.e. good shot feel and good controllability.

[0006] According to the present invention, the object described above has been accomplished by employing a thread wound core which comprises a solid center composed of an inner center and a center outer layer, and a thread rubber layer formed on the solid center, and adjusting a diameter of the inner center and the solid center and a specific gravity difference between the inner center and center outer layer to a specified range, thereby providing a solid center type thread wound golf ball having long flight distance, while maintaining the characteristics peculiar to the conventional thread wound golf ball, i.e. good shot feel and good controllability.

[0007] This object as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description with reference to the accompanying drawings.

SUMMARY OF THE INVENTION

[0008] The present invention provides a solid center type thread wound golf ball comprising

[0009] (a) a solid center composed of an inner center and a center outer layer formed on the inner center,

[0010] (b) a thread rubber layer formed on the solid center, and

[0011] (c) a cover covering the thread rubber layer, wherein the inner center has a diameter of 20 to 35 mm, the solid center has a diameter of 30 to 38 mm, and a specific gravity of the center outer layer is larger than that of the inner center by 0.1 to 0.4.

[0012] In order to put the present invention into a more suitable practical application, it is desired that the inner center have a center hardness in JIS-C hardness of 30 to 70, the center outer layer have a surface hardness in JIS-C hardness of 70 to 90, and the cover has a Shore D hardness of 40 to 65.

BRIEF EXPLANATION OF DRAWINGS

[0013]FIG. 1 is a schematic cross section illustrating one embodiment of the golf ball of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] The solid center type thread wound golf ball of the present invention will be explained with reference to the accompanying drawing. FIG. 1 is a schematic cross section illustrating one embodiment of the golf ball of the present invention. As shown in FIG. 1, the golf ball of the present invention comprises a solid center 5 which is composed of an inner center 1 and a center outer layer 2, a thread rubber layer 3 and a cover 4 formed on the thread rubber layer 3. The inner center 1 and the center outer layer 2 will be explained together, because the both are formed from the same material. The both are formed from a rubber composition comprising a base rubber, a co-crosslinking agent, an organic peroxide, a filler and optionally an antioxidant.

[0015] The base rubber may be natural rubber and/or synthetic rubber, which has been conventionally used for solid golf balls. Preferred is high-cis polybutadiene rubber containing a cis-1,4 bond of not less than 40%, preferably not less than 80%. The polybutadiene rubber may be mixed with natural rubber, polyisoprene rubber, styrene-butadiene rubber, ethylene-propylene-diene rubber (EPDM), and the like.

[0016] Examples of the co-crosslinking agents include a metal salt of α,β-unsaturated carboxylic acid, particularly mono- or di-valent metal salts, such as zinc or magnesium salts of α,β-unsaturated carboxylic acids having 3 to 8 carbon atoms (e.g. acrylic acid, methacrylic acid, etc.). Preferred co-crosslinking agent is zinc acrylate because it imparts high rebound characteristics to the resulting golf ball. The amount of the co-crosslinking agent may be 5 to 26 parts by weight, preferably 8 to 25 parts by weight in the inner center 1, and 26 to 40 parts by weight, preferably 27 to 35 parts by weight in the center outer layer 2, based on 100 parts by weight of the base rubber. When the amount of the co-crosslinking agent is larger than 26 parts by weight in the inner center 1, or 40 parts by weight in the center outer layer 2, the center is too hard, and the shot feel of the resulting golf ball is poor. On the other hand, when the amount of the co-crosslinking agent is smaller than 5 parts by weight in the inner center 1, or 26 parts by weight in the center outer layer 2, the center is too soft, and the rebound characteristics are degraded, which reduces the flight distance.

[0017] Examples of the organic peroxides, which acs as a crosslinking agent or hardener, include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di (t-butylperoxy)-hexane, di-t-butyl peroxide and the like. Preferred organic peroxide is dicumyl peroxide. The amount of the organic peroxide is from 0.5 to 2.0 parts by weight, preferably 0.8 to 1.5 parts by weight, based on 100 parts by weight of the base rubber. When the amount of the organic peroxide is smaller than 0.5 parts by weight, the center is too soft, and the rebound characteristics are degraded, which reduces the flight distance. On the other hand, when the amount of the organic peroxide is larger than 2.0 parts by weight, the center is too hard, and the shot feel is poor.

[0018] Examples of the fillers, which can be used for the core of the golf ball, include for example, inorganic filler (such as zinc oxide, barium sulfate, calcium carbonate, and the like), high specific gravity metal powder filler (such as tungsten powder, molybdenum powder, and the like), and the mixture thereof. The amount of the filler in the inner center 1 is from 5 to 40 parts by weight, preferably 7 to 35 parts by weight, based on 100 parts by weight of the base rubber. When the amount of the filler is smaller than 5 parts by weight, the inner center is too light, and it is required to add large amount of filler to the center outer layer 2. Therefore, the rebound characteristics of the center outer layer 2 are degraded, which reduces the flight distance of the resulting golf ball. On the other hand, when the amount of the filler is larger than 40 parts by weight, the specific gravity of the inner center is larger than that of the center outer layer, and the moment of inertia of the resulting golf ball is small, which reduces the flight distance. The amount of the filler in the center outer layer 2 is from 30 to 55 parts by weight, preferably 33 to 50 parts by weight, based on 100 parts by weight of the base rubber. When the amount of the filler is smaller than 30 parts by weight, the specific gravity of the center outer layer is smaller than that of the inner center 1, and the moment of inertia of the resulting golf ball is small, which reduces the flight distance. On the other hand, when the amount of the filler is larger than 55 parts by weight, the weight of the core is too large, and the weight of the resulting golf ball is too large.

[0019] The rubber composition for the inner center 1 and the center outer layer 2 of the present invention can contain other components, which have been conventionally used for preparing the core of solid golf balls, such as antioxidant or peptizing agent. If used, the amount of the antioxidant is preferably 0.2 to 0.5 parts by weight, based on 100 parts by weight of the base rubber.

[0020] The inner center 1 is obtained by mixing the rubber composition, followed by vulcanizing or press-molding the mixture in a mold. In the thread wound golf ball of the present invention, it is required for the inner center 1 to have a diameter of 20 to 35 mm, preferably 25 to 34 mm, more preferably 29 to 34 mm. When the diameter of the inner center is smaller than 20 mm, the amount at the time of hitting increases, which reduces the flight distance. On the other hand, when the diameter of the inner center is larger than 35 mm, the center outer layer is too thin. Therefore, the rebound characteristics as the technical effects of the center outer layer do not sufficiently exhibit, which reduces the flight distance.

[0021] The inner center 1 has a center hardness in JIS-C hardness of 30 to 70, preferably 40 to 65. The JIS-C hardness is substantially the same as Shore C hardness. When the hardness is smaller than 30, the rebound characteristics of the resulting golf ball are degraded, which reduces the flight distance. When the hardness is larger than 70, the shot feel is hard and poor. In addition, the spin amount increases, which reduces the flight distance. The term “center hardness of an inner center” as used herein refers to the hardness, which is determined by cutting the inner center into two equal parts and then measuring a JIS-C hardness at its center point in section.

[0022] The inner center 1 has a specific gravity of 1.00 to 1.50, preferably 1.10 to 1.40. When the specific gravity is smaller than 1.00, it is required to add large amount of filler to the center outer layer 2 in order to obtain a suitable weight of the golf ball. Therefore, the rebound characteristics of the center outer layer 2 are degraded, which reduces the flight distance of the resulting golf ball. On the other hand, when the specific gravity is larger than 1.50, the specific gravity of the solid center is large, and it is required to reduce the diameter of the solid center in order to obtain a suitable weight of the golf ball. Therefore, the spin amount at the time of hitting is large, and thus the golf ball creates blown-up trajectory, which reduces the flight distance. The center outer layer 2 is then formed on the inner center 1.

[0023] The center outer layer of the present invention may be formed by conventional methods, which have been known to the art and used for the cover of the golf balls. For example, there can be used a method comprising molding the center outer layer composition into a semi-spherical half-shell, covering the inner center with the two of the half-shells, followed by pressure molding, or a method comprising injection molding the center outer layer composition directly on the inner center to cover it. As described above, the center outer layer 2 is formed on the inner center 1 to obtain the solid center 5.

[0024] It is required for the solid center 5 of the golf ball of the present invention to have a diameter of 30 to 38 mm, preferably 31 to 37 mm. When the diameter is smaller than 30 mm, the spin amount of the resulting golf ball increases, which reduces the flight distance. On the other hand, when the diameter is larger than 38 mm, the thread rubber layer is too thin. Therefore, the rebound characteristics as the technical effects of the thread rubber layer do not sufficiently exhibit, which reduces the flight distance.

[0025] It is desirable for the center outer layer 2 of the golf ball of the present invention to have a surface hardness in JIS-C hardness of 70 to 90, preferably 72 to 85. When the hardness is smaller than 70, the resulting golf ball is too soft, and the shot feel is faint and poor. When the hardness is larger than 90, the resulting golf ball is too hard, the shot feel is hard and poor. The term “surface hardness of a center outer layer 2” as used herein refers to the hardness, which is determined by measuring a JIS-C hardness at the surface of the solid center, which obtained by forming the center outer layer on the inner center.

[0026] The center outer layer 2 has a specific gravity of 1.20 to 1.60, preferably 1.30 to 1.50. When the specific gravity is smaller than 1.20, the specific gravity of the solid center is small, and it is required to increase the diameter of the solid center in order to obtain a suitable weight of the golf ball. Therefore, the thread rubber layer is too thin, and the rebound characteristics as the technical effects of the thread rubber layer do not sufficiently exhibit, which reduces the flight distance. On the other hand, when the specific gravity is larger than 1.60, it is required to add large amount of filler to the center outer layer 2. Therefore, the rebound characteristics of the center outer layer 2 are degraded, which reduces the flight distance of the resulting golf ball.

[0027] In the golf ball of the present invention, it is required that the specific gravity of the center outer layer be larger than that of the inner center by 0.1 to 0.4, preferably 0.14 to 0.35, more preferably 0.14 to 0.26. The specific gravity difference is smaller than 0.1, the technical effect of increase the moment of inertia is sufficiently obtained, which reduces the flight distance. On the other hand, when the specific gravity difference is larger than 0.4, it is required to add large amount of filler to the center outer layer 2. Therefore, the rebound characteristics of the center outer layer 2 are degraded, which reduces the flight distance of the resulting golf ball.

[0028] Molding the solid center within the ranges described above can be conducted by adjusting the formulation of the inner center and the center outer layer. That is, it can be conducted by adjusting the amount of the filler in the rubber composition so that the amount in the center outer layer is larger than that in the inner center to obtain the desired specific gravity difference. When the amount of the filler in the center outer layer is too large, the rebound characteristics are degraded. Therefore, it is preferable in the center outer layer to use a high specific gravity metal powder such as tungsten powder or mixtures with the inorganic filler in place of the inorganic filler. The thread rubber layer 3 is then formed on the solid center 5.

[0029] The thread rubber used for winding around the solid center 5 may be of the same kind which is conventionally used in thread-wound layers in thread-wound golf balls; e.g., it can be obtained by vulcanizing a rubber composition in which natural rubber or natural rubber and synthetic polyisoprene have been compounded with sulfur, a vulcanization aid, vulcanization accelerator, antioxidant and the like. The thread rubber layer 3 is formed on the solid center by conventional methods which have used for preparing the thread wound core of the thread wound golf balls. The thread rubber layer 3 has a thickness of 1.0 to 5.0 mm, preferably 1.2 to 4.5 mm. When the thickness of the thread rubber layer is smaller than 1.0 mm, the thread rubber layer is too thin to exhibit sufficient impact relaxation, and the shot feel is poor. On the other hand, when the thickness is larger than 5.0 mm, the spin amount at the time of hitting increases, which reduces the flight distance reduces. The cover 4 is then formed on the thread rubber layer 3.

[0030] The cover 4 of the golf ball of the present invention is formed from a thermoplastic resin, particularly ionomer resins, which are known to the art and have been used for the cover of golf balls, as a base resin. The ionomer resin used in the present invention is not limited, but includes a copolymer of α-olefin and α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms, of which a portion of carboxylic acid groups is neutralized with metal ion; a terpolymer of α-olefin, α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms and α, β-unsaturated carboxylic acid ester having 2 to 22 carbon atoms, of which at least a portion of carboxylic acid groups is neutralized with metal ion; or mixtures thereof. In the cover of the golf ball of the present invention, the ionomer resin, of which 5 to 80% of carboxylic acid groups are neutralized with metal ion, may be used. The α-olefin in the ionomer resin is preferably ethylene or propylene. Examples of the α, β-unsaturated carboxylic acid in the ionomer, for example, include acrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid and the like. Preferred are acrylic acid and methacrylic acid. Examples of the α, β-unsaturated carboxylic acid ester in the ionomer, for example, include methyl ester, ethyl ester, propyl ester, n-butyl ester and isobutyl ester of acrylic acid, methacrylic acid, fumaric acid, maleic acid and the like. Preferred are acrylic acid esters and methacrylic acid esters. Examples of the metal ions, which neutralizes a portion of carboxylic acid groups of the copolymer or terpolymer, include a sodium ion, a potassium ion, a lithium ion, a magnesium ion, calcium ion, a zinc ion, a barium ion, an aluminum ion, a tin ion, a zirconium ion, a cadmium ion and the like. Preferred are sodium ion, zinc ion, magnesium ion and the like, in view of rebound characteristics, durability and the like.

[0031] The ionomer resin is not limited, but examples thereof will be shown by trade names. Examples of the ionomer resin, which is commercially available from Mitsui Du Pont Polychemical Co., include Hi-milan 1555, Hi-milan 1557, Hi-milan 1605, Hi-milan 1652, Hi-milan 1702, Hi-milan 1705, Hi-milan 1706, Hi-milan 1707, Hi-milan 1855, Hi-milan 1856 and the like. Examples of the ionomer resins, which is commercially available from Du Pont Co., include Surlyn 8945, Surlyn 9945, Surlyn AD8542 and the like. Examples of the ionomer resin, which is commercially available from Exxon Chemical Co., include Iotek 7010, Iotek 8000, and the like. The ionomers may each be used alone or in combinations of two or more.

[0032] As the materials used in the cover 4 of the present invention, the above ionomer resin may be used alone, but a heat mixture of the ionomer resin and thermoplastic elastomer may be used. Examples of the thermoplastic elastomers include:

[0033] (a) maleic anhydride-modified thermoplastic elastomers,

[0034] (b) epoxy group-modified thermoplastic elastomers, such as block copolymers comprising SBS (styrene-butadiene-styrene) structure with epoxy groups or block copolymers comprising SIS (styrene-isoprene-styrene) structure with epoxy groups,

[0035] (c) thermoplastic elastomers having terminal OH groups, such as block copolymers comprising SEBS (styrene-ethylene-butylene-styrene) structure; or block copolymers comprising SEPS (styrene-ethylene-propylene-styrene) structure,

[0036] or combinations thereof. The amount of the ionomer resin is from 20 to 80% by weight, and the amount of the thermoplastic elastomer is from 80 to 20% by weight, based on the total weight of the base resin of the cover.

[0037] Examples of maleic anhydride-modified thermoplastic elastomers include ethylene-ethyl acrylate-maleic anhydride terpolymers, which are commercially available from Sumitomo Chemical Co., Ltd., under the trade name of “Bondine” (such as “Bondine AX8390”) and the like.

[0038] Examples of epoxy group-modified thermoplastic elastomers, which contain epoxy groups in elastomer molecule, include glycidyl methacrylate adducts of hydrogenated SBS block copolymers, which are commercially available from Asahi Chemical Industry Co., Ltd. under the trade name of “Tuftec Z514”, “Tuftec Z513” and the like; SBS block copolymers comprising polybutadiene block with epoxy groups, which are commercially available from Daicel Chemical Industries Co., Ltd. under the trade name of “Epofriend A1010”, “Epofriend A1005”, “Epofriend A1020”; and the like.

[0039] Examples of block copolymers comprising SEBS structure or SEPS structure having terminal OH groups include hydrogenated styrene-isoprene-styrene (SIS) block copolymers having terminal OH groups, which is commercially available from Kuraray Co., Ltd. under the trade name of “Septon HG-252” and the like.

[0040] In the golf ball of the present invention, the resin composition for the cover may optionally contain fillers (such as barium sulfate, etc.), pigments (such as titanium dioxide, etc.), and the other additives (such as dispersants, antioxidants, UV absorbers, photostabilizers and fluorescent agents or fluorescent brighteners, etc.), in addition to the resin component, as long as the addition of the additives does not deteriorate the desired performance of the golf ball cover, but an amount of the pigment is preferably from 1.0 to 6.0 parts by weight based on 100 parts by weight of the cover base resin.

[0041] The cover of the present invention may be formed by conventional methods, which have been known to the art and used for forming the cover of the golf balls. For example, there can be used a method comprising molding the cover composition into a semi-spherical half-shell in advance, covering the thread wound core with the two half-shells, followed by pressure molding at 130 to 170° C. for 1 to 5 minutes, or a method comprising injection molding the cover composition directly on the thread wound core to cover it.

[0042] It is desired for the cover 4 of the present invention to have a Shore D hardness of 40 to 65, preferably 40 to 60, more preferably 45 to 58, most preferably 50 to 57. When the hardness is smaller than 40, the cover is too soft, and the rebound characteristics of the resulting golf ball are degraded, which reduces the flight distance. On the other hand, when the hardness is larger than 65, the spin amount at the time of hitting by an iron club decreases, and the controllability is degraded. In addition, the impact force at the time of hitting is large, and the shot feel is poor. In the golf ball of the present invention, it is possible to maintain the excellent controllability when hit by an iron club by adjusting the cover hardness, and to accomplish the golf ball having long flight distance when hit by a driver and excellent controllability when hit by an iron club by the combination of the cover with the solid center having the technical effect of improving the flight distance as described above.

[0043] It is desired for the cover to have a thickness of 1.0 to 3.5 mm, preferably 1.5 to 3.0 mm. When the thickness of the cover is smaller than 1.0 mm, the cover is easy to break when repeatedly hitting, and the durability is degraded. On the other hand, when the thickness is larger than 3.5 mm, the spin amount decreases, which degrades the controllability when hit by an iron club, or the shot feel is poor. At the time of molding the cover, many depressions called “dimples” may be optionally formed on the surface of the golf ball. Furthermore, paint finishing or marking with a stamp may be optionally provided after the cover is molded for commercial purposes.

EXAMPLES

[0044] The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope of the present invention.

[0045] (i) Production of Inner Center

[0046] Each inner center was obtained by mixing the rubber composition for the inner center having the formulation shown in Table 1 (Examples) and Table 2 (Comparative Examples) and press-molding the mixture at 160° C. for 20 minutes. The diameter, center hardness in JIS-C hardness and specific gravity (A) of the resulting inner center were measured, and the results are shown in Table 4.

[0047] (ii) Production of Solid Center

[0048] The rubber compositions for center outer layer having formulations shown in Table 1 (Examples) and Table 2 (Comparative Examples) were molded into semi-spherical half-shells, encapsulating the above inner center with the two half-shells, followed by press-molding in a mold at 160° C. for 20 minutes to obtain solid center. The diameter, surface hardness in JIS-C hardness and specific gravity (B) of the resulting solid center were measured, and the results are shown in Table 4. The specific gravity difference between the inner center and center outer layer (B-A) was obtained by calculating and the results are shown in the same Table. TABLE 1 (parts by weight) Example No. Center composition 1 2 3 4 5 (Inner center composition) BR11 *1 100 100 100 100 100 Zinc acrylate 10 15 15 15 20 Zinc oxide 15 15 15 15 15 Dicumyl peroxide 1.2 1.2 1.2 1.2 1.2 Antioxidant *2 0.5 0.5 0.5 0.5 0.5 Barium sulfate 31 29.5 29.5 33.5 10 (Center outer layer composition) BR11 *1 100 100 100 100 100 Zinc acrylate 27 30 30 30 33 Zinc oxide 15 15 15 15 15 Dicumyl peroxide 1.2 1.2 1.2 1.2 1.2 Antioxidant *2 0.5 0.5 0.5 0.5 0.5 Tungsten 48 38.5 38.5 36.5 41

[0049] TABLE 2 (parts by weight) Comparative Example No. Center composition 1 2 3 (Inner center composition) BR11 *1 100 100 100 Zinc acrylate 15 10 10 Zinc oxide 15 15 15 Dicumyl peroxide 1.2 1.2 1.2 Antioxidant *2 0.5 0.5 0.5 Barium sulfate 40.5 40 78.5 (Center outer layer composition) BR11 *1 100 100 100 Zinc acrylate 30 27 23 Zinc oxide 15 15 15 Dicumyl peroxide 1.2 1.2 1.2 Antioxidant *2 0.5 0.5 0.5 Tungsten 27.5 40.5 80

[0050] Formation of Thread Rubber Layer

[0051] Each thread rubber layer was then formed on the solid center by winding the thread rubber, of which a base rubber is a blend of natural rubber and a low cis-isoprene rubber (“Shell IR-309” commercially available from Shell Chemical Co., Ltd.)=50/50 (weight ratio) to obtain a thread wound core. The thread rubber was prepared from as a base rubber. A diameter of the thread wound core was 39.0 mm.

[0052] Preparation of Cover Composition

[0053] The formulation materials shown in Table 3 were mixed using a kneading type twin-screw extruder to obtain pelletized cover compositions. The extrusion condition were

[0054] a screw diameter of 45 mm,

[0055] a screw speed of 200 rpm, and

[0056] a screw L/D of 35.

[0057] The formulation materials were heated at 200 to 260° C. at the die position of the extruder. The Shore D hardness of the resulting cover compositions was shown in Table 4. The Shore D hardness was determined according to ASTM D-2240, using a sample of a stack of the three or more heat and press molded sheet having a thickness of about 2 mm from each composition, which had been stored at 23° C. for 2 weeks. TABLE 3 (parts by weight) Cover composition A B C D Hi-milan 1605 *3 15 — — 50 Hi-milan 1706 *4 15 — — 50 Hi-milan 1855 *5 70 20 — — Surlyn 8945 *6 — 25 25 — Surlyn 9945 *7 — 25 25 — Tuftec Z514 *8 — 20 — — Bondine AX8390 *9 — 10 — — Epofriend A1010 *10 — — 15 — Septon HG-252 *11 — — 35 — Titanium dioxide  2  2  2  2 Barium sulfate  2  2  2  2

Examples 1 to 5 and Comparative Examples 1 to 3

[0058] The resulting cover compositions were molded into semi-spherical half-shells, encapsulating the resulting thread wound core with the two half-shells, followed by press-molding in the mold for golf ball and then coating with a paint to obtain a thread wound golf ball having an outer diameter of 42.8 mm and a weight of 45.4 g. With respect to the resulting golf balls, the flight performance (initial velocity, launch angle, spin amount and carry) and shot feel were measured or evaluated, and the results are shown in Table 5. The test methods are as follows.

[0059] Test method

[0060] (1) Flight performance

[0061] After a No. 1 wood club (a driver, W#1) having a metal head was mounted to a swing robot manufactured by True Temper Co. and a golf ball was hit at head speed of 45 m/sec, the initial velocity, launch angle, spin amount and flight distance were measured. The spin amount was measured by continuously taking a photograph of a mark provided on the hit golf ball using a high-speed camera. As the flight distance, carry that is a distance to the drop point of the hit golf ball was measured.

[0062] After a No. 9 iron club (I#9) was mounted to a swing robot manufactured by True Temper Co. and a golf ball was hit at head speed of 34 m/sec, the spin amount was measured as described above.

[0063] The measurement was conducted 5 times for each golf ball (n=5), and the average is shown as the result of the golf ball. It is required when hitting by an iron club, it is required for golf balls to have excellent spin performance. It is desired for golf balls to have a spin amount when hitting by a No. 9 iron club of not less than 7000 rpm, preferable not less than 8000 rpm. When the spin amount is smaller than 7000 rpm, the spin amount is too small, and the controllability is poor.

[0064] Golf club used for testing the flight performance

[0065] W#1: “Hi-brid Autofocus” loft angle: 10 degrees, shaft hardness: S, manufactured by Sumitomo Rubber Industries, Ltd.

[0066] I#9: “Hi-brid Autofocus” shaft hardness: S, manufactured by Sumitomo Rubber Industries, Ltd.)

[0067] (2) Shot Feel

[0068] The shot feel of the golf ball is evaluated by 10 golfers according to a practical hitting test using a driver having a metal head. The evaluation criteria are as follows. The results shown in the Tables below are based on the fact that the most golfers evaluated with the same criterion about shot feel.

[0069] oo: The golfers felt that the golf ball has small impact force, and good shot feel.

[0070] o: The golfers felt that the golf ball has fairly good shot feel.

[0071] x: The golfers felt that the golf ball has large impact force, and poor shot feel. TABLE 4 Comparative Example No. Example No. Test item 1 2 3 4 5 1 2 3 (Inner center) Diameter 24 28 28 28 32 28 18 24 (mm) Center 40 50 50 50 60 51 40 40 hardness (JIS-C) Specific 1.25 1.25 1.25 1.27 1.15 1.31 1.25 1.50 gravity A (g/cm³) (Center outer layer) Surface 75 80 80 80 85 80 75 72 hardness (JIS-C) Specific 1.45 1.39 1.39 1.37 1.41 1.31 1.40 1.68 gravity B (g/cm³) Difference 0.20 0.14 0.14 0.10 0.26 0 0.15 0.18 (B-A) (Solid center) Diameter 32 34 34 34 36 34 32 28 (mm) (Cover) Composition A C D C B C C C Shore D 55 52 63 52 54 52 52 52 hardness

[0072] TABLE 5 Comparative Example No. Example No. Test item 1 2 3 4 5 1 2 3 Flight performance I (W#1, 45 m/sec) Initial 65.0 64.9 65.2 64.9 64.7 64.9 65.1 65.2 velocity (m/sec) Spin 2850 2780 2680 2890 2720 2880 3120 3350 amount (rpm) Carry (m) 204.3 205.0 206.0 203.2 204.0 199.5 199.2 197.8 Flight performance II (I#9, 34 m/sec) Spin 8600 8550 7220 8590 8480 8580 8600 8680 amount (rpm) Shot feel oo oo o oo oo oo oo oo

[0073] As is apparent from Table 4 and Table 5, the golf balls of the present invention of Examples 1 to 5 had longer flight distance than the conventional thread wound golf balls of Comparative Examples 1 to 5. In the golf balls of Examples 1, 2, 4 and 5, the impact force at the time of hitting is small, and the shot feel is good. In the golf ball of Example 3, the cover hardness is slightly large, and the spin amount is slightly small and the shot feel is slightly poor, but the flight distance is very long, when compared with the other Examples.

[0074] To the contrary, in the golf ball of Comparative Example 1, the moment of inertia is small, and the flight distance is short, because the specific gravity difference is too small. In the golf ball of Comparative Example 2, the spin amount is large, and the flight distance is short, because the diameter of the inner center is too small. The golf ball of Comparative Example 3, the spin amount is large, and the flight distance is short, because the diameter of the solid center is too small. 

What is claimed is:
 1. A solid center type thread wound golf ball comprising (a) a solid center composed of an inner center and a center outer layer formed on the inner center, (b) a thread rubber layer formed on the solid center, and (c) a cover covering the thread rubber layer, wherein the inner center has a diameter of 20 to 35 mm, the solid center has a diameter of 30 to 38 mm, and a specific gravity of the center outer layer is larger than that of the inner center by 0.1 to 0.4.
 2. The solid center type thread wound golf ball according to claim 1 , wherein the inner center has a center hardness in JIS-C hardness of 30 to 70, and the center outer layer has a surface hardness in JIS-C hardness of 70 to
 90. 3. The solid center type thread wound golf ball according to claim 1 , wherein the cover has a Shore D hardness of 40 to
 65. 4. The solid center type thread wound golf ball according to claim 1 , wherein the solid center is formed from a rubber composition comprising polybutadiene containing not less than 40% of a cis-1, 4 bond, a metal salt of α, β-unsaturated carboxylic acid and an organic peroxide.
 5. The solid center type thread wound golf ball according to claim 1 , wherein the thread rubber layer has a thickness of 1.0 to 5.0 mm.
 6. The solid center type thread wound golf ball according to claim 1 , wherein the cover has a thickness of 1.0 to 3.5 mm. 